Do we know enough of the workings of string theory to say what factors give rise to a large or small value of the velocity of propagation of massless fields for a given multiverse?
The speed of light is derivative because it is has units. What's fundamental is the fine structure constant. If that's different then the speed of massless objects is differentI guess the speed of light has a definition within an individual universe, with respect to whatever "rods and clocks" (as in special relativity) exist there.
The speed of light is derivative because it is has units.